Characterising the agents of major solar system isotopic heterogeneity

Rocky planets and asteroids are almost completely formed from of the condensed debris of dying stars (star dust) that has been ejected into the inter-stellar medium. Very many individual stars contribute to this ever-changing mixture, in a process termed Galactic Chemical Evolution. The composition of our solar system represents a snap-shot of this evolution path in our corner of the galaxy at the time of the formation of the sun.

The many different stellar explosions that contribute the bulk composition of our solar system add elements with radically different isotopic signatures (isotopes are atoms of the same element but with different masses), which fingerprint a variety of element generating processes (nucleosynthesis) that occur in different sized stars. However, fine grained star dust is rapidly homogenised in subsequent processing so the isotopic composition of the solar system shows remarkably little variability given this diverse input.

Nonetheless, subtly different isotopic signatures have been detected in different meteorites that represent fragments of planetary bodies formed at variable distances from the Sun. Such differences are inferred to result from the products stellar explosions that occurred close in time to the formation of the solar system, so that there was not enough time to for the distinct star dust to be comminuted.

Most notable amongst solar system isotopic variability are the unusually neutron rich isotopes of the elements Ca, Ti and Cr. Yet these isotopes are believed to be made only made in rare stellar explosions that are unexpected to detonate close to the birth of the solar system. Thus finding the nature of the carriers of these isotopic anomalies is of great interest to test ideas about how they formed.

Perhaps more importantly, if we can identify the grains that carry these signatures and determine their physical properties (size, composition, density, mineralogy) this would provide us with the information to test what processes can sort these grains to produce the isotopic variability seen in the solar system.

Thus much is to be gained by finding the hosts of neutron-rich Ti and Ca, but currently there have only been glimpses of their presence. This is maybe unsurprising as there are considerable analytical challenges in identifying tiny anomalous grains (typically only 200 nanometres across) within a 'sea' of normal sample matrix. In this project we will undertake a comprehensive search for neutron-rich carriers using complementary , sophisticated techniques that can map isotopic compositions at resolutions from a few microns to tens of nanometres.

Once we have learnt how to identify the grains in situ within meteorites, we will use micro-manipulation techniques to remove individual pre-solar grains and examine them under a high-resolution electron micro-scope before final analysis by laser ablation. As a result we will characterise the size, composition and isotopic signatures of the grains that shape bulk compositional variability across the solar system.